Question

The reason is that的用法，为什么？

Answer 1

the reason is that 是由that 引导的表语从句,that位于be动词之后,也做引导词,不能省略.例：The reason is that he has had a headeache.(原因是他头痛.）

is后不可以接副词或是副词性从句,because引导的是原因状语从句,所以应该是the reason is that.

the reason is because意思是:原因是.

The reason is that意思是: 理由是.

Answer 2

The reason is that...,that引导的是一个表语从句，表语的内容是去解释主语，即the reason（原因）就是后面的从句意思。因此that既不能换成because也不能换成why